Monitoring of gas sensors

ABSTRACT

A monitor is disclosed for monitoring an atmosphere for the presence of a target gas, the monitor comprising:  
       1.  an electrochemical gas sensor ( 11 ) having a working (sensing) electrode ( 11   a ) and a counter electrode ( 11   b ), the sensor providing a current between the electrodes that is indicative of the amount of target gas in the atmosphere;  
       2.  an operational amplifier ( 12 ) connected between the sensor electrodes to generate an output signal according to the current flowing between the terminals, whereby the output signal is indicative of the amount of target gas in the atmosphere,  
       3.  a detector ( 20,22 ) for detecting when the current flowing between the sensor electrodes exceeds a predetermined threshold; and  
       4.  a circuit ( 24 ) that restricts the potential difference between the sensor electrodes when the current between the terminals exceeds the predetermined threshold by supplying additional current to or removing current from the working sensor electrode ( 11   a ).

TECHNICAL FIELD

[0001] The present invention relates to the monitoring of sensors thatare used for detecting and measuring quantities of gases or vapours inan ambient atmosphere. The present specification will refer to suchsensors as “gas sensors”, although throughout the present specificationthat term also applies to the measurement of vapours. The presentinvention is especially concerned with the monitoring of electrochemicalgas sensors, e.g. for measuring toxic gases such as carbon monoxide.

STATE OF THE ART

[0002] One use of gas sensors is to monitor the level of carbon monoxidein a dwelling, particularly in temporary accommodation. Various nationaland international standards apply to the detection of carbon monoxide indwellings, including a requirement to measure with reasonable accuratelyup to 600 parts per million (ppm) of carbon monoxide.

[0003] After exposure to a large concentration of gas, the reading tendsnot to return to zero when the toxic gas is no longer present in theatmosphere. Instead, at zero gas, there is a negative reading and thesensor only returns to accurate operation after a substantial recoverytime, which can be of the order of days. It would be desirable to reducethe recovery time. In addition, standards are set governing recoverytime. For example, CENELEC EN 50192 requires domestic carbon monoxidesensors to respond to carbon monoxide levels up to 50 ppm within onehour of being exposed to a carbon monoxide concentration of 5,000 ppm.

[0004] Commercial pressures require such carbon monoxide sensors to berelatively inexpensive; electrochemical sensors include electrodescarrying expensive catalyst, which is usually a metal from the platinumgroup (Group VIII metal). One way of reducing the cost of such sensorsis to restrict the amount of such catalyst used. If a relatively smallamount of catalyst is used, the speed of recovery of a sensor from anexposure to a large gas concentration is slow.

[0005] The standards applying to carbon monoxide sensing equipment(called herein “monitors”) also requires an alarm signal to be generatedif the sensor is faulty, for example if the sensor is not properlyconnected to the electronic circuitry within the monitor or if thesensor has dried out (i.e. lost sufficient volume of electrolyte) or ifthere is a short circuit between the terminals of the sensor.

[0006] It is known to test the viability of an electrochemical gassensor by imposing an electric pulse across it; U.S. Pat. No. 5,202,637discloses a three electrode sensor that can be monitored by applying apulse of potential between the reference electrode and the sensingelectrode (also known as the working electrode). Although current doesnot flow at a significant level between the electrodes, the pulsecharges up the double ionic layer at the sensing electrode and thisresults in a current flow in external circuitry, which can be detectedto show that the sensor is operational. Obviously, if the sensor hasdried out or if there is a poor connection between the sensor and thecircuit, no current will flow and an “error” signal can be generated.

[0007] Unfortunately, it is not always possible to detect the pulse whenthere is a large concentration of gas in the atmosphere being monitoredsince it can be swamped by the signal from the gas.

[0008] In EP-0840112, a sensor is connected to the inverting terminal ofan operational amplifier while a voltage pulse is applied periodicallyto the non-inverting terminal of the amplifier. In normal operation theoperational amplifier is acting as a transimpedance amplifier, the gainof which is defined by V_(out)/I_(input) and given by the value of thefeedback resistor between the negative input and output of theoperational amplifier. If the sensor should become short circuited, theoperational amplifier will become a high gain voltage amplifier whoseoutput is the product of the open loop gain and the input offsetvoltages of the inputs of the operational amplifier. Within low costoperational amplifiers used in domestic types of gas monitors, theseparameters can be poorly controlled and as a result the output can beany value including an apparently valid gas reading or an over-rangecondition. Accordingly, when a pulse is applied to the non-invertingterminal, the amplifier it is not able to determine if the sensor isexposed to an over range high gas concentration or if the sensor hasbecome short circuited.

DISCLOSURE OF INVENTION

[0009] According to the present invention, there is provided a monitorfor monitoring an atmosphere for the presence of a target gas, themonitor comprising:

[0010] a) two terminals for connection, respectively, to the working(sensing) electrode and the counter electrode of an electrochemical gassensor, the sensor providing a current between the terminals that isindicative of the amount of target gas in the atmosphere;

[0011] b) an operational amplifier connected between the sensorelectrode terminals to generate an output signal according to thecurrent flowing between the terminals, whereby the output signal isindicative of the amount of target gas in the atmosphere,

[0012] c) a detector for detecting when the current flowing between thesensor terminals exceeds a predetermined threshold; and

[0013] d) a circuit that restricts the potential difference between thesensor electrode terminals when the current between the terminalsexceeds the predetermined threshold by supplying additional current toor removing additional current from the working sensor terminal.

[0014] The detector may detect when the current flowing between thesensor terminals exceeds a predetermined threshold directly orindirectly, for example by monitoring the output signal of theoperational amplifier and generating a signal when the amplifier outputsignal exceeds a threshold value (e.g. when the amplifier output signalis saturated).

[0015] The circuit that adjusts the current at the working sensorterminal when the current between the terminals exceeds a predeterminedthreshold may be an active component such as a transistor between theterminals of the sensor, the circuit reducing the resistance of thetransistor when the detector detects an excessive current, therebyallowing current to flow between the sensor terminals. Alternatively,the current may comprise a current source (if the working electrode isan anode) or a current drain (if the working electrode is a cathode) andan active component such as a transistor, e.g. a FET, to connect thecurrent source to the working electrode, thereby reducing the potentialdifference across the sensor terminals.

[0016] The present invention also provides a method of monitoring anatmosphere for the presence of a target gas using the monitor asdescribed above.

DESCRIPTION OF DRAWINGS

[0017] The present invention will now be described, by way of exampleonly, with reference to the accompanying drawings in which:

[0018]FIG. 1 is a schematic circuit diagram showing a prior art circuit;

[0019]FIG. 2 is a graph showing the output of a gas monitor over timewhen exposed to a high concentration of carbon monoxide.

[0020]FIG. 3 is a schematic circuit diagram showing the operation of thepresent invention;

[0021]FIG. 4 is a further schematic circuit diagram showing a circuit ofthe present invention; and

[0022]FIG. 5 is a detailed circuit diagram of the circuit of FIG. 4.

BEST METHOD FOR CARRYING OUT THE INVENTION

[0023] Referring initially to FIG. 1, there is shown a known gasmonitoring circuit having a two-electrode gas sensor 10, the structureof which is well known (see EP-0,840,112). Essentially, the sensorincludes a sensing electrode 11 a and a counter electrode 11 b separatedby an intervening body of electrolyte. The sensing electrode 11 a isexposed to the atmosphere being monitored and accordingly any toxic gas(in this carbon monoxide) in the atmosphere comes into contact with thesensing electrode 11 a. The sensing electrode 11 a is an anode andoxidises the carbon monoxide to carbon dioxide. This oxidation causes acurrent to flow through the sensor between the working electrode 11 aand the counter 11 b. A resistor 16 is connected between the sensingelectrode 11 a and the inverting input of an operational amplifier 12.The non-inverting input of the amplifier is connected to the earth 14.The amplifier has a negative feedback including a resistor 17. Thetransimpedance gain on the operational amplifier 12 is about 125,000fold.

[0024] The presence of carbon monoxide at the sensing electrode 11 acauses the sensing electrode to generate a current proportional to theamount of gas present. The potential difference between the electrodesfloats until it reaches a level that is sufficient to generate thecurrent concerned. The current generated at the sensing electrode causesa potential difference across the resistor 16, causing a change ofpotential at the inverting input of the amplifier 12. The operationalamplifier 12 generates a signal at its output that is proportional tothe potential between its inputs and so the output signal isproportional to the current flowing in the sensor 10 and hence theamount of gas in the atmosphere being monitored. The output signal canbe fed to a display and an alarm (neither shown) to display theconcentration of carbon monoxide in the atmosphere and to generate analarm if the concentration exceeds a pre-set threshold. Alternatively,the integrated concentration can be computed over different time periodsto generate alarms based on the rate that the human body absorbs aparticular concentration of gas.

[0025] By providing a negative feedback, the amplifier attempts tomaintain a fixed offset potential (usually zero) between its inputs. Thesize of the feedback current is proportional to the output signal. Inthese circumstances, the potential difference between the sensing andreference electrodes fluctuates within a relatively narrow range.

[0026]FIG. 2 is a graph showing the signal at the amplifier output 15against time when the sensor is exposed to a substantial level of carbonmonoxide in the atmosphere being monitored. The signal rises rapidlyuntil time t₁ where the signal is saturated as it reaches a plateau P.At time t₂ the carbon monoxide is removed from the atmospheresurrounding the sensor and the current falls. However, it does not fallto a zero signal but overshoots. If the operational amplifier isoperated from a split supply rail the output would go negative for aperiod until it recovers. However, these circuits are typically run froma single supply and so the output would be zero, even in the presence ofa certain amount of carbon monoxide. The “negative” signal is probablydue to the chemistry in the cell altering when the potential between theelectrodes is high.

[0027] It can take some considerable time for the signal to return to azero value when in contact with an atmosphere free of carbon monoxide.CENELEC require a cell to recover within one hour after exposure to 5000ppm carbon monoxide. This may be difficult to achieve when the sensingand counter electrodes contain a relatively small amount of catalyst,which is desirable commercially in order to reduce its cost.

[0028] We have discovered that if the load across the sensor 10 isreduced when passing high currents, particularly when the operationalamplifier is saturated, the sensor will recover more quickly. By “load”,we mean the requirement on the cell to increase the potential betweenits electrodes when exposed to more target gas in the atmosphere beingmonitored in order to pass more current. If the working electrode is ananode, this reduction in load can be achieved by injecting additionalcurrent to the working electrode which, together with the current fromthe amplifier feedback circuit, provides the current that the cellrequires to oxidise all the target gas it is in contact with. If theworking electrode is a cathode, excessive current may be drained fromthe working electrode if the amplifier feedback circuit cannot drain allthe current generated by the working cathode.

[0029] In this specification, the term “current” is used in theconventional electrical sense, i.e. current flows in the oppositedirection to the electron flow.

[0030] The variation in the load can be achieved in several ways. FIG. 3shows one method of varying the load; components illustrated in FIG. 3that are identical to those illustrated in FIG. 1 are indicated by thesame reference number.

[0031] In FIG. 3, a field effect transistor (FET) 18 is included betweenthe working and counter electrodes of the sensor 10. It usually has avery high resistance between its drain and source so that little or nocurrent flows through it and the circuit operates in the same way asdescribed in connection with FIG. 1. The amplifier output 15 isconnected via an analogue-to-digital converter 20 to a microprocessor22, which monitors the output signal on output 15. If the output signalis saturated, i.e. reaches a threshold level, the current supplied bythe feedback resistor is limited; in these circumstances, themicroprocessor 22 generates a signal to a digital-to-analogue converter24 which reduces the resistance of the FET 18. This allows a current toflow from the counter electrode 11 b to the working electrode 11 awhich, together with the current supplied through the feedback resistor17, supplies the full current required by the working electrode tooxidise all the carbon monoxide molecules that diffuse into contact withthe working electrode, The addition of additional current prevents thepotential between the electrodes in the sensor 10 from increasingmarkedly in order to pass the current required by the concentration ofCO in contact with the working electrode 11 a.

[0032] The microprocessor 22 continues reducing the resistance of theFET 18 until the output signal of the amplifier is no longer saturated.Once that state of affairs has been reached, the microprocessor 22periodically increases the resistance of the FET 18 until the output isagain saturated, whereupon it promptly reduces the resistance again toachieve an output signal just below the saturation level. If theconcentration of CO in the atmosphere reduces, the microprocessor willreturn the circuit to its usual operational state, in which theresistance of FET 18 is high, automatically using the above operatingregime.

[0033] While the resistance of the FET 18 is in a reduced state, theoutput signal of the amplifier 12 will not give a measure of the amountof gas in the atmosphere. The reduction in the resistance 18 across thesensor 10 can be measured and used to give an indication of the amountof gas in the atmosphere being monitored, as follows. The microprocessor22 will control the resistance of FET 18 to an extent to bring theoutput signal 15 down to a predetermined level just below saturation.The amount of gas needed to cause the signal output 15 to be saturatedis known. The reduction of resistance of FET 18 can be correlated withthe amount of gas in the atmosphere. Accordingly, the reduction in theresistance gives an indication of the amount of gas in the atmosphereover and above the amount of gas required to maintain a signal at thesaturated level. Although this generally will not be a particularlyaccurate measure, nevertheless, it is useful. The resistance reductioncan be calculated, for example, by the microprocessor 22.

[0034] A circuit (not shown) is known that applies a pulse of potentialacross the sensor to monitor the viability of the sensor. If the sensoris viable, a pulse in the output circuit is produced that can bedetected. However the output pulse will be difficult to detect if theamplifier output is saturated By reducing the amplifier output to belowsaturation, in accordance with the present invention, it will still bepossible to monitor the sensor since the output will no longer besaturated at high gas concentrations and so the pulse in the outputsignal as a result of the pulse of potential applied between theelectrodes of the sensor can still be detected.

[0035] Finally, the output signal will take a shorter time to recoverafter an exposure to a high gas concentration. Typically the output ofthe sensor to 50 ppm gas would be reduced to only 40 to 50% of itsnormal reading following an exposure to 5000 ppm for 15 minute followedby 60 minutes in clean air. With this technique the output response to50 ppm following the same process would typically be 85 to 95% ofnormal.

[0036] Instead of responding to a saturated amplifier output signal, themicroprocessor 22 can be set to respond to a lower signal, i.e. when thesignal is at a threshold below saturation.

[0037] An alternative circuit is shown in FIG. 4 (again the componentsalready described will be indicated with the same reference numbers).The FIG. 4 circuit differs from the FIG. 3 circuit in that no FET 18 isprovided and instead, the digital-to-analogue converter 24 is connectedto the monitor power supply, e.g. a battery (not shown), via a powersupply rail 26. Also, the resistor 16 in FIG. 3 is split into twoseparate resistors 16 a and 16 b.

[0038] In the FIG. 4 circuit, if the microprocessor 22 detects that theoutput of the amplifier is saturated, the digital-to-analogue converter24 injects current from the power rail 26 into the sensor circuit atpoint 19 between the two resistors 16 a and 16 b. The injected currentI_(s), together with the current I_(m) supplied through the feedbackresistor 17, supplies the full current I_(c) required by the workingelectrode 11 a to oxidise all the carbon monoxide molecules that diffuseinto contact with the working electrode. Once the amplifier output 15 issaturated, the microprocessor 22 increases the amount of currentinjected from rail 26 until the output signal of the amplifier is nolonger saturated. Once that has been achieved, the microprocessor 22periodically decreases the injected current until the output is againsaturated, whereupon it increases the current again to achieve anamplifier output signal below the saturation level. If the concentrationof CO in the atmosphere reduces, the microprocessor 22 will return thecircuit to its usual operational state (with no current being injected)automatically using the above operating regime.

[0039] The circuit of FIG. 4 has the same advantages as described abovein connection with FIG. 3.

[0040] A working circuit corresponding to the schematic circuit of FIG.4 will now be described in connection with FIG. 5. The components shownin both FIGS. 4 and 5 are indicated by the same reference numbers.

[0041] The microprocessor 22 includes a square wave generator 22′ (PWMOutput) that is connected to the base of a transistor Q1 that isconnected also to the supply rail 26 of the monitor. Resistor R1 andcapacitor C3 provide a low frequency filter that filters out thefrequency of the square wave and so a voltage is applied to the base ofthe transistor that is the weighted average of the peaks and troughs ofthe applied square wave, i.e. if the peaks and troughs are of equalduration, the voltage applied to the gate will be half that voltage ofthe peak voltage and if the peaks are of much longer than the troughs,the voltage supplied to the base is a little less than the voltage ofthe supply. The transistor Q1 acts as an emitter follower so that thevoltage of the emitter is 0.7 volts less than that applied to the gateof Q1. In this way, the voltage applied by the transistor Q1 to a diodeD3 can be set by altering the ratio of the durations of the peaks andtroughs from the square wave generator 22′. The emitter of transistor Q1is connected via diode D3 and a resistor R3 to a point 33 and so thecurrent supplied to the point 33 from the transistor Q1 can becontrolled by adjusting the ratio of the peaks of the square wave fromgenerator 22′ to the troughs.

[0042] Section 32 of the circuit is a charge pump providing a constantvoltage of −3 volts at point 31 of the circuit. Section 34 containingtransistors Q2 and Q3 is a constant current source providing a currentof −30 μA to point 33 at the voltage of point 31, i.e. −3V, irrespectiveof the peaks and troughs of the square wave generator 22. By controllingthe square wave generator to produce no peaks for a short period, nocurrent is applied to point 33 by transistor Q1 and so the currentflowing at point 33 and hence at point 19 will be −30 μA. By controllingthe square wave generator to provide an appropriate proportion of peaks,the transistor Q1 can be made to supply a current of +60 μA at point 33,which results in a current of +30 μA being supplied to the point 19. Inthis way, successive positive and negative pulses of 30 μA and −30 μAcan be applied to point 19. If the square wave contains an appropriateproportion of peaks, the transistor Q1 can be made to supply a currentof +30 μA at point 33, which cancels the current from sections 32 and 34and hence no current flows to point 19. If a current of, for example+330 μA, is supplied by the transistor Q1 under the control of thesquare wave generator 22′, then a current of +300 μA is supplied to thepoint 19. In this way, the current supplied to point 19 can becontrolled and the circuit will operate as already described inconnection with FIG. 4.

[0043] The circuit of FIG. 5 can be used to apply pulses periodically tothe sensor to detect whether it is viable. This is achieved by thetransistor Q1, under the control of the microprocessor 22, supplyingvirtually no current to point 33 causing a pulse of −30 μA to be appliedto point 19 by the charge pump and constant current source 32,34. Afterabout 600 milliseconds a current of +60 μA is supplied by transistor Q1to point 33 causing a pulse of +30 μA to be applied to point 19. After afurther 600 milliseconds, the output of the transistor Q1 returns to thenormal state of affairs described above. The double pulses can beapplied periodically, e.g. every minute, to ensure that the sensor isfunctioning properly. The proper functioning of the sensor is detectedby a change in the signal over the course of a pulse caused by thecurrent pulses charging or discharging the charge on the workingelectrode. If the working electrode is not functioning properly or thesensor is not connected properly (or at all) or if there is a shortcircuit across the sensor, the signal will not change so much (or atall) over the course of the pulse and so this is indicative of a faultin the monitor's functioning.

[0044] In the case of a CO sensor 10, the normal state of affairs willbe for the square wave generator to be dormant, i.e. it does notgenerate any square waves. This removes the power supply to transistorQ1 and the charge pump 32 and hence no current will be supplied at point19. If however, a saturated signal is generated by amplifier 12, thiswill be detected by the microprocessor 22 and the square wave generatorwill be reactivated and, under the control of the wave generator 22′, anappropriate current can be injected at point 19 to bring the outputsignal below the saturation level.

[0045] As already discussed, there is a tendency for the output of thesensor to produce a current in the opposite direction from that normallyproduced (i.e. when there is target gas in the atmosphere beingmonitored) following exposure to high gas concentrations and subsequentremoval of the gas, i.e. conventional current flows out of the workinganode electrode. By injecting short pulses of −30 μA into the cell tooppose this current, the time for the sensor cell to recover normaloperation is decreased. Thus if the output of the sensor cell isnegative, which in a single rail monitor means that the output of theoperational amplifier is zero, such short pulses of current will assistin bringing the sensor back to its normal operational state morequickly.

[0046] The circuit of FIG. 5 can also be used to detect a short circuitin a different way. As stated above, the signal produced when the sensoris short circuited can be any value, depending on the offset voltage ofthe operational amplifier 12. Usually, however, the output signal of theoperational amplifier will be saturated and hitherto it has beenimpossible to tell that condition from the condition in which there isan excessive amount of gas in the atmosphere. However, if there is anexcessive amount of gas in the atmosphere, the circuit of FIG. 5 will beable to bring the amplifier output down to a level in which it is notlonger saturated, as described above, but if there is a short circuit,it will not be able to and so a signal indicating that there is a shortcircuit can be generated in these circumstances.

[0047] In practice, the ADC 20, the microprocessor 22 (including thesquare wave generator 22′) and the DAC 24 are all part of onemicroprocessor chip.

1: A monitor for use with a 2-terminal sensor for monitoring anatmosphere for the presence of a target gas, the monitor comprising: a)two terminals for connection, respectively, to the working (sensing)electrode and the counter electrode of an electrochemical gas sensor,the sensor providing a current between the terminals that is indicativeof the amount of target gas in the atmosphere, said monitor having onlytwo contacts for connection to the 2-terminal sensor; b) an operationalamplifier connected between the sensor electrode terminals to generatean output signal according to the current flowing between the terminals,whereby the output signal is indicative of the amount of target gas inthe atmosphere, c) a detector for detecting when the current flowingbetween the sensor terminals exceeds a predetermined threshold; and d) acircuit that restricts the potential difference between the sensorelectrode terminals when the current between the terminals exceeds thepredetermined threshold by supplying additional current to or removingcurrent from the working sensor terminal. 2: A monitor as claimed isclaim 1, wherein the detector detects when the current flowing betweenthe sensor terminals exceeds a predetermined threshold by monitoring theoutput signal of the operational amplifier and generates a signal whenthe amplifier output signal exceeds a threshold value. 3: A monitor asclaimed is claim 2, wherein the detector detects when the amplifieroutput signal is saturated. 4: A monitor as claimed in claim 1, whereinthe circuit that adjusts the current at the working sensor terminal whenthe current between the terminals exceeds the predetermined thresholdcomprises a variable resistance device, e.g. a FET, connected betweenthe terminals of the sensor, the circuit reducing the resistance of theresistor when the detector detects an excessive current, therebyallowing current to flow between the sensor terminals. 5: A monitor asclaimed in claim 1, wherein the circuit that adjusts the current at theworking sensor terminal when the current between the terminals exceedsthe predetermined threshold comprises a current source and a switch(which is preferably an active component such as a transistor) toconnect the current source to the working electrode, thereby reducingthe potential difference between the sensor terminals. 6: A monitor asclaimed in claim 1, wherein the circuit that adjusts the current at theworking sensor terminal when the current between the terminals exceedsthe predetermined threshold comprises a current drain and a switch(which is preferably an active component such as a transistor) toconnect the current drain to the working electrode, thereby reducing thepotential difference across the sensor terminals. 7: A monitor asclaimed in claim 1, which includes a 2-terminal electrochemical gassensor connected to the terminals. 8: A method of monitoring anatmosphere for the presence of a target gas by means of a 2-terminalelectrochemical gas sensor having a working (sensing) electrode and acounter electrode, the sensor providing a current between the electrodesthat is indicative of the amount of target gas in the atmosphere, themethod comprising: a) detecting when the current flowing between thesensor terminals exceeds a predetermined threshold; and b) restrictingthe potential difference between the sensor terminals when the currentbetween the terminals exceeds the predetermined threshold by supplyingadditional current to or removing current from the working sensorterminal. 9: A method as claimed in claim 8, wherein additional currentis supplied to or current is removed from the working sensor terminal instep b) by providing a bypass circuit between the working and thecounter electrodes that includes a resistor and reducing the resistanceof the bypass circuit so that current flows between the working and thecounter electrodes. 10: A method as claimed in claim 8, whereinadditional current is supplied to the working sensor terminal in step b)by injecting current from a current source. 11: A method as claimed inclaim 8, wherein current is removed from the working sensor terminal instep b) via a current drain. 12: A method of increasing the rate ofrecovery of a 2-terminal electrochemical gas sensor having a working(sensing) electrode and a counter electrode that produces a negativecurrent (i.e. a current flowing in the direction opposite to thatprevailing when, in normal operation, it detects gas in the atmospherebeing monitored), the method comprising a) detecting when a negativecurrent is flowing in the sensor and b) when a negative current isflowing in the sensor, applying a pulse of current between theelectrodes of the sensor that is opposite to the said negative current.